The present invention relates to an optical projection exposure method and a system using the same and, more particularly, a method and system for exposing a micropattern such as an LSI pattern to form the micropattern on a substrate using an optical projection lens.
An optical projection exposure system called a stepper for forming a micropattern such as an LSI pattern conventionally requires a high resolving power. In order to satisfy this demand, an optical projection lens of a recent optical projection exposure system has a larger numerical aperture and shortens the wavelength of light to improve a resolution property. The most advanced optical projection lens has a resolving power almost close to a theoretical limit determined by a wavelength of light. Nevertheless, a higher resolving power is required to cope with micropatterning of recent LSI patterns. In order to cope with this demand, there is proposed a phase-shifting mask method in which phase sifters are added every predetermined interval of transparent patterns so that a phase difference .pi. appears in transmitted rays between adjacent patterns on the mask placed on the object plane of the optical projection lens, thereby causing a light intensity in an opaque region to come close to zero. This method will be described by way of a simple example.
FIG. 17 is a view for explaining a resolution limit of the mask of this phase-shifting mask method, while FIG. 18 is a view for explaining a resolution limit of a mask by a normal illumination method. Each mask is assumed to have a large number of line figures having equal widths and parallel to each other at equal intervals. FIG. 18 is a sectional view of a mask M taken along a plane perpendicular to a direction of lines of the mask M. A length of a repetition period of the line figures of the mask M shown in FIG. 18 is defined as d. Phase shifters S are added every predetermined interval of transparent regions in a phase-shifting mask MP shown in FIG. 17 to cause a phase difference .pi. between rays passing through two adjacent transparent regions. That is, since a negative amplitude is obtained every predetermined interval of the patterns, the period is defined as 2d, and a DC component becomes zero. For this reason, illumination light I.sub.0 having a wave number k.sub.0 and vertically incident on the phase-shifting mask MP is diffracted by the mask to produce a wave I.sub.1 inclined at an angle with respect to an optical axis z. The wave number of this wave is defined as k.sub.1 =k.sub.0 sin(.alpha.') where .alpha.' is a diffraction angle. However, since the fundamental period of the repetition pattern is 2d, the wave number is defined as k.sub.1 =2.pi./2d. An electric field amplitude u of transmitted light has components in .+-.x directions and is represented as follows: ##EQU1## where x is a coordinate in a pattern repetition direction.
When a wave defined by equation (1) propagates toward the optical projection lens, it is assumed that this wave passes near the periphery of an aperture A (entrance pupil) located below the phase-shifting mask MP. That is, assume that a small pattern diffracted at an angle larger than the size of the aperture A cannot be resolved by the optical projection lens. Since a light intensity in image plane is proportional to a square of an absolute value of the electric field amplitude, the following equation is derived: EQU .vertline.u.vertline..sup.2 =(1/2).vertline.u.sub.0 .vertline..sup.2 (1+cos2k.sub.1 x) (2)
so that a pattern having a fundamental period d and a wave number k.sub.1 can be reproduced.
On the other hand, when the light I.sub.0 having the wave number k.sub.0 is vertically incident on the normal mask M having no phase shifter S, a wave I.sub.1 ' having the following wave number is produced, as shown in FIG. 18: EQU k2k.sub.1 =2k.sub.0 sin(.alpha.')
This wave I.sub.1 is shielded by the aperture A and does not reach the image plane. That is, in vertical incidence on the normal mask, the pattern is not resolved.
As can be apparent from the above description, a resolving power can be increased as compared with the conventional technique when the phase-shifting mask method is used.
As described above, although the phase-shifting mask method is effective for adjacent linear patterns, it is always not effective for adjacent patterns having different sizes and isolated patterns. A step of adding phase shifters in mask fabrication results in a low yield and high mask fabrication cost.
Another method satisfying a higher resolving power to cope with micropatterning is a ring light source aperture method. As described in Japanese Patent Application No. 59-211269 (Japanese Patent Laid-Open No. 61-91662 laid open on May 9, 1986) entitled the "Optical Projection Exposure System" and filed by the present applicant, a ring aperture is used as a light source aperture. The function of this method is based on experimental facts. This prior art describes that a higher resolving power can be obtained by an outermost light source in the ring light source aperture. However, no description is made on a method of determining a size of the ring light source aperture to obtain a maximum resolution of the optical projection lens.